Thousands of people from around Haiti have gathered in a park in the capital for the state funeral of former president Rene Preval.

Mourners marched beneath the hot sun and chanted "long live Preval" as roving traditional rara bands performed festival music at the Champ de Mars plaza in Port-au-Prince, where Preval's flag-draped coffin was displayed.

Three former Haitian presidents - Michel Martelly, Jocelerme Privert and Boniface Alexandre - attended the funeral along with foreign diplomats and representatives of international organizations.

Mr Preval was the only democratically elected president to win and complete two terms in Haiti.

An agronomist by training, Mr Preval kept a low profile in office even in the aftermath of the devastating earthquake in 2010, which led some to accuse him of not showing leadership at a time of crisis.

But mourners had only kind words for him on Saturday.

Haitian President Jovenel Moise hailed Mr Preval as a "leader for all Haitians who showed respect to everyone. Let's continue to work to make Haiti a better place".

Mario Prasmars, 42, said he travelled 18 hours to attend the funeral, calling Mr Preval "a father for Haiti".

In the afternoon, the casket was travelling four hours to the rural northern town of Marmelade, where he was born.

Family members have said that Mr Preval, who died on March 3, had a heart attack on the way to the hospital. The 74-year-old former leader had health problems including prostate cancer in the past.

Attorney General Donton Leger said this week that a post mortem examination did not clearly establish a cause of death and more tests would be required. He acknowledged rumours that the former president had been poisoned, without citing evidence.

Mr Preval was elected by a landslide in 1995 as the chosen successor of Jean-Bertrand Aristide, who has not commented publicly on his death and did not attend the funeral.

His second term was characterised at the start by a sharp rise in kidnappings and widespread hunger due to higher food prices followed by the earthquake. The government said the disaster killed about 310,000 people but the precise death toll is unknown.